Interpacking

1. In sand which contains particles in a variety of sizes, a process of compaction which can reduce the permeability of the sand.

Sand Backfill should be of the correct particle size to prevent migration into the granular backfill. A narrow particle-size distribution is very important because like-sized particles do not inter-pack and result in good stable porosity in the soil.
Source: http://iog.icampus.ac/vl/read_chapter.php?learn_id=5896